Various kinds of printing processes such as flexographic printing, letter press printing, gravure printing, screen printing and offset printing are used for packaging materials such as corrugated boards, paper containers, paper bags and flexible packaging films, building and decoration materials such as wallpapers and decorative plates, label printing and the like. Particularly, flexographic printing, or letter press printing using a resin letterpress having a relatively high hardness has an increased proportion among a various kinds of printing processes. For fabrication of printing plates for use in these printing processes, normally, photosensitive resins are often used. For example, a process in which a liquid photosensitive resin or a solid photosensitive resin plate in the form of a sheet is used, a photo mask is placed on the photosensitive resin, light is applied through the mask to cause a crosslinking reaction, and non-crosslinked parts are then washed out with a developing solution has been used. In recent years, so called a flexographic CTP technique of providing a thin light absorption layer called a black layer on the surface of a photosensitive resin, applying laser light to the layer to form a mask image directly on a photosensitive resin plate, then applying light through the mask to cause a crosslinking reaction, and then washing out non-crosslinked parts of areas to which no light is applied has been developed and increasingly employed in terms of the effect of improving efficiency of fabrication of a printing plate. However, this technique has a limited effect of improving the efficiency, and has a problem of absolutely requiring a developing step, and a technique in which a relief image is formed directly on a printing original plate using laser and no developing step is required is desired.
One process for solving the problem is a process in which a printing original plate is directly subjected to laser engraving. Fabrication of letterpress printing plates and stamps by this process has already been conducted. As materials for use in this process, materials prepared by heating and curing synthetic rubbers such as EPDM and silicones have already been used. However, not only these materials require much time for production because it takes much time for heating and curing to achieve necessary mechanical properties, aging is further required until the properties become stable, and so on, but also the former materials using synthetic resins and the like as raw materials have a disadvantage that sculpture debris is stuck on the surface of a plate when the plate is subjected to laser engraving, and it is so difficult to remove the debris that a precise image cannot be formed, and the latter materials using silicones as raw materials have a disadvantage that the speed of laser engraving is so low that it takes much time to fabricate a plate, the resistance to a solvent ink is low, and so on.
As a process for overcoming the disadvantages of the materials described above, a process for producing a laser engraving flexographic printing plate, in which laser light is applied to a cured photosensitive resin obtained by photo-curing a photosensitive resin composition to form a concavo-convex pattern on the surface, has been proposed.
For example, Patent Document 1 (Japanese Patent No. 2846954 (U.S. Pat. No. 5,798,202)) and Patent Document 2 (Japanese Patent No. 2846955 (U.S. Pat. No. 5,804,353)) disclose use of a material prepared by mechanically, photochemically and thermochemically reinforcing a thermoplastic elastomer such as SBS, SIS or SEBS.
Patent Document 3 (JP-A-56-64823) and Patent Document 4 (JP-A-2002-79645) disclose a process for forming a roll material prepared by photo-curing a liquid photosensitive resin. Further, the present inventors have proposed in Patent Document 5 (WO 03/022594 A) a laser engraving printing original plate using a liquid photosensitive resin composition containing a polymeric material that is a plastomer at 20° C., and proposed a process allowing improvements in which an inorganic porous material is made to coexist to inhibit generation of liquid debris caused by a laser, tack on the surface of the plate is inhibited, and an optical system is prevented from being contaminated. The present inventors have proposed in Patent Document 6 (PCT/JP2004/005839) an apparatus for producing a laser engraving printing original plate using a liquid photosensitive resin composition and a process for producing an original plate using the apparatus.
For a laser engraving process using a cured photosensitive resin, it is highly expected that high-quality printing can be performed as compared to a case where a conventional printing plate by light exposure and development is used because of the advantage that the edge of a formed pattern becomes sharp and thickening of an image by a printing pressure is inhibited, and further, very fine dots can be formed taking advantage of the characteristic such that the focal point of laser light is small, and completion of the technique is strongly desired.
Owing to advancement of a laser engraving apparatus in recent years, it is being made possible to form a sharp edge and very fine dots, but it has become evident that when this is to be achieved in actual printing, there is a problem that should further be solved. More specifically, if the edges of narrow lines and dot parts are sharp as one example, thickening of an image by a printing pressure surely tends to be inhibited, but deformation by the printing pressure tends to occur at the same time, and therefore the sharpened edges do not lead to faithful reproduction in printing. Accordingly, further improvements in mechanical properties of a material are required.
Although the edge of an image on a plate becomes so sharp that fine patterns can be formed, it is not sufficient for achieving high-quality printing, and it is extremely important that the printing plate has mechanical properties such that the accuracy of its plate thickness can be ensured. In the case of a process by laser engraving, a photosensitive resin is first cured before formation of an image by sculpture, and the surface is then subjected to surface processing such as cutting, grinding, polishing and the like, whereby higher thickness accuracy can be ensured as compared to a printing plate by light exposure and development which is generally used today. As a result of examining surface processing processes such as cutting, grinding and polishing further in detail using the aforementioned conventional photosensitive resin composition, the present inventors have found that since sticky debris generated in these processes is deposited on the surface, and deposited sticky debris is hard to be removed, cutting and polishing marks may remain on the surface, and the processing accuracy of the obtained surface becomes so insufficient that a problem arises in printing quality especially when performing precise printing. The above described sticky debris is deposited on the surface of a cutting bite or a grinding wheel, and clings in some cases. It has been found that for avoiding these problems, carefully performing processing, such as stopping working to clean out or remove deposited debris when the sticky debris clings to the cutting bite or the grinding wheel, is required, and therefore a considerable amount of time is required for processing. In addition, as a problem when using a plate of a precise image, it often becomes a serious problem that the image tends to fall off at the time of wiping the plate, or the like.
Thus, it is necessary to improve the mechanical strength so that high-resolution printing as described above can be performed without increasing the hardness of the printing plate because if flexographic printing while making full use of the advantages of good image formability of the letterpress printing plate by laser engraving.
For example, the aforementioned Patent Document 1 (Japanese Patent No. 2846954 (U.S. Pat. No. 5,798,202)) and Patent Document 2 (Japanese Patent No. 2846955 (U.S. Pat. No. 5,804,353)) disclose use of a material prepared by mechanically, photochemically and thermochemically reinforcing a thermoplastic elastomer such as SBS, SIS or SEBS, and provide that the elastomer is mechanically strengthened by adding a filler. However, mixing the filler cannot solve the aforementioned problems, since the hardness of the material is increased at the same time, and so on.
Patent Documents 3 to 6 do not describe the aforementioned problems. Patent Document 4 (JP-A-2002-79645) describes a process for forming a seamless cylinder printing plate for laser engraving, in which in a step of applying light to a liquid photosensitive resin to photo-cure the resin, a light source emitting mainly light having a wavelength of 200 nm to 400 nm is used. As specific light sources, a wide range of light sources including light sources having a relatively high intensity, such as chemical lamps, bactericidal ray lamps, high pressure mercury lamps and metal halides lamps and light sources having a relatively low intensity are listed. However, application of light having a specific high illuminance is not mentioned at all.
As described above, when the laser engraving technique is to be made practicable so that a still higher level of performance is achieved, there has not been a technique allowing mechanical properties to be improved without affecting the hardness of the printing plate among previous material design techniques.
Patent Document 1: Japanese Patent No. 2846954
Patent Document 2: Japanese Patent No. 2846955
Patent Document 3: JP-A-56-64823
Patent Document 4: JP-A-2002-79645
Patent Document 5: WO 03/022594 A
Patent Document 6: PCT/JP2004/005839 Pamphlet